1. Field of the Invention
The present invention generally relates to processes for increasing production yield during slurry polishing in semiconductor manufacturing and more particularly to processes which prevents solids from forming in the slurry.
2. Description of the Related Art
A common form of planarizing semiconductor substrates is chemical mechanical polishing (CMP). Chemical mechanical polishing, as its name suggests, utilizes both of the physical operation associated with polishing and the reactions associate with chemical compounds, to planarize a surface of a semiconductor device.
More specifically, as illustrated and FIG. 1, in chemical mechanical polishing, a delivery tube 10 supplies a slurry 11 to a polishing pad 12. The polishing pad 12 is rubbed against a surface to be planarized, such as a semiconductor substrate (not illustrated).
An important element of chemical mechanical polishing is the abrasive slurry 11. To maintain uniform polishing, the abrasive particles within the slurry 11 should remain consistently suspended throughout all areas between the polishing pad and the surface being polished. If the abrasives within the slurry join together and form solids, the surface being polished can be easily scratched by the solids.
As illustrated in FIG. 2, the force of gravity (indicated by the arrow 20) causes particles within the slurry 11 traveling within the delivery tube 10 to accumulate at the bottom of the delivery tube 10. This situation commonly occurs between polishing cycles, or at other times when the flow of the slurry 11 within the delivery tube 10 is stopped or slowed.
Indeed, the accumulation of the abrasive components and other particles within the slurry 11 may settle out and accumulate when a filter, such as a cartridge filter 30 illustrated in FIG. 3, is utilized. Such filters 30 are commonly designed to remove solids from the slurry. However, cartridge filters decrease the velocities of the slurry 11, which encourages an accumulation of particles, as is illustrated in area 31 of the cartridge filter 30.
When particles are permitted to accumulate, two major types of larger particles are formed. One type of gel-like accumulation is referred to as "flocculate" and a second type of harder accumulation of particles is referred to as an "agglomerate", both of which are illustrated in FIG. 4. Normal representations of dispersed particles are also illustrated in FIG. 4, for reference.
Conventionally, the problem of particle accumulation was solved by agitating the supply tank (not illustrated) which feeds the delivery tube 10. However, as mentioned above, when the velocity within the delivery tube 10 is reduced or stopped, particle accumulation will continue to occur regardless of the amount of agitation within the supply tank.
Additional conventional production techniques used to avoid the accumulation of solids includes the use of filters, such as the cartridge filter 30 shown in FIG. 3. However, as mentioned above, such a filter can actually promote particle accumulation because filters tend to reduce the velocity of the slurry within the delivery tube 10.
Further, filters simply stop solids and do not provide mechanisms for returning solids as dispersed particles to the slurry. Also, filters can become clogged and further reduce the velocity of the slurry, which, again, promotes particle accumulation.
For example, in the slurry for tungsten, the abrasive components will settle out and can flocculate/agglomerate if there is insufficient agitation. This situation exists in the delivery tube when, for example, the flow is interrupted during processing. The same problem occurs when filtering is attempted because the fluid velocity is too slow to agitate the suspension. The particle settling causes a variation in the solid content of the slurry as it is delivered. This variation in content results in a rate variation that degrades process control. As mentioned above, agglomerated slurry may contribute to scratching defects.
Conventional attempts to reduce solids within the delivery tube also include placing a filter at the very end of the tube which delivers the slurry to the pad 12. Such a filter is referred to as a "point of use" filter. In oxide polishing slurries, it has been found that point of use filters worked very well at removing large particles, thus helping to improve yield. Filters are also used in the recirculation loop (not illustrated) to remove gel particles that form as the slurry ages. Again, such a filter helps reduce foreign material defects on the product and increases projected yields.
Similar attempts have been made to use "point of use" filters on metal polishing slurries to remove large particles contaminating or forming within the slurry just before dispensing the slurry on the polishing pad. Unfortunately, metal polishing slurries, such as tungsten polishing slurry, are not colloidally stable and settle out of solution in the point of use filter.
Another conventional solution to the problem of solid formation involves the use of a very short delivery system without filtering. While such a system helps prevent the accumulation and variation in slurry composition, it does nothing to eliminate large particles and the growth of particles that contribute to scratch defects.
An additional conventional problem that occurs when the velocity is changed suddenly is that the particles which have previously settled out of the slurry, tend to rejoin the flow and increase the abrasive content being delivered. In other words, the conventional abrasive content is varied according to time and slurry flow history.